1. Field of the Invention
The present invention relates to an interferometric measuring device having a short-coherent radiation source and a system of a modulation interferometer and a downstream reference interferometer coupled thereto.
2. Description of Related Art
A conventional interferometric measuring device is described in published German patent document DE 102 44 553, which discloses an interferometric measuring device for detecting the shape, the roughness, or the distance of the surface of a measured object using a modulation interferometer, to which short-coherent radiation is supplied by a radiation source and which has a first beam splitter for splitting the supplied radiation into a first partial beam conducted via a first arm and a second partial beam conducted via a second arm, the light phase or light frequency of one of the partial beams being shifted with respect to the other by a modulation device and this beam passing through a delay segment, the two partial beams being subsequently combined in a further beam splitter of the modulation interferometer, having a measuring probe spatially separated from the modulation interferometer and connected or connectable thereto via a fiber optic device in which the combined partial beams are split into a measuring beam conducted to the surface by a probe fiber optic device having an oblique object-side exit surface and a reference beam and in which the measuring beam (r1(t)) reflected on the surface and the reference beam (r2(t)) reflected on a reference plane are superimposed, and having a receiving device and an analyzing unit for converting the radiation supplied thereto into electric signals and for analyzing the signals on the basis of a phase difference. The angle of inclination (y) of the exit surface is at least 46° with respect to the normal to the optical probe axis.
Published German patent document DE 198 08 273 also describes such an interferometric measuring device for detecting the shape or distance, in particular of rough surfaces, having at least one spatially coherent beam generating unit whose radiation is split, in a measuring probe, into a measuring reference beam conducted through a measuring reference branch and reflected therein and a measuring beam reflected on the rough surface, having a device for modulating the light phase or for shifting the light frequency (heterodyne frequency) of a first partial beam with respect to the light phase or the light frequency of a second partial beam, having a superposition unit for superimposing the reflected measuring reference beam on the reflected measuring beam, having a beam decomposition and beam receiving unit for splitting the superimposed radiation into at least two beams having different wavelengths and for converting the radiation into electric signals, and having an analyzing unit in which the shape and/or distance of the rough surface is determinable on the basis of a phase difference of the electric signals. The radiation emitted by the beam generating unit is short-coherent in time and broadband.
Such interferometric measuring devices initially having two interferometers may be designed to have different interferometer types. The modulation interferometer may thus be designed as a Mach-Zehnder interferometer, while the measuring interferometer or the measuring probe may be designed as a compact interferometer, for example, a Mirau interferometer. The interferometric measuring devices share the feature that the path difference between two partial beams, established in the first interferometer, is equalized again by a short-coherent radiation source in the second measuring interferometer or measuring probe and the partial beams are thus brought to form interference. The path difference established by a delay element in published German patent document DE 198 08 273 may also be produced by partial arms of different lengths, through which the partial beams pass, as in a modulation interferometer featuring optical fibers depicted in published German patent document DE 198 08 273.
In order to improve the measuring accuracy of the interferometric measuring device, it is known to connect a reference interferometer to a second output of the modulation interferometer. It has an optical design like that of the measuring interferometer, i.e., it again equalizes the path difference between the two partial beams established in the modulation interferometer. However, the design of the reference interferometer differs from that of the measuring interferometer. The measuring accuracy of the interferometric measuring device may be improved by comparing the signals of the reference interferometer with those of the measuring interferometer.
The path difference to be established in the modulation interferometer depends on the design of the measuring interferometer or the measuring probe. After replacement of the measuring interferometer/measuring probe and/or a channel switch in the case of probes having multiple outputs, the path difference in the modulation interferometer must be adjusted accordingly. This is usually accomplished by a servo-motor by displacing optical components.
The path difference must be adjusted accordingly also in the reference interferometer. Here the path difference is usually adjusted by replacing a pre-adjusted unit or by using dual-channel reference probes having fixedly preset distances. The disadvantage here is that units adjusted to the measuring interferometer used must be available. The replacement of units is complicated.
An object of the present invention is to provide an interferometric measuring device which makes simple and highly accurate adjustment of the path differences established between partial beams of optically coupled interferometers possible.
It is furthermore an object of the present invention to provide a method therefor.